Porcine reproductive and respiratory syndrome (PRRS) has devastated the global swine industry causing immense economic losses (Neumann, E. J., J. B. Kliebenstein, C. D. Johnson, J. W. Mabry, E. J. Bush, A. H. Seitzinger, A. L. Green, and J. J. Zimmerman. 2005. Assessment of the economic impact of porcine reproductive and respiratory syndrome on swine production in the United States. J Am Vet Med Assoc 227:385-92). Severe outbreaks continue to appear periodically worldwide. For example, a highly pathogenic pig disease (“pig high fever disease”) caused by a variant strain of PRRSV recently emerged in China with 20-100% mortality. New and more severe PRRS outbreaks will likely continue to emerge worldwide. The causative agent, PRRSV, is a small, enveloped, single-stranded, positive-sense RNA virus in the family Arteriviridae (Snijder, E. J., and J. J. Meulenberg. 1998. The molecular biology of arteriviruses. J Gen Virol 79 (Pt 5):961-79). At least seven structural proteins are translated from a 3′-coterminal nested set of subgenomic mRNAs of PRRSV (Meng, X. J., P. S. Paul, I. Morozov, and P. G. Halbur. 1996. A nested set of six or seven subgenomic mRNAs is formed in cells infected with different isolates of porcine reproductive and respiratory syndrome virus. J Gen Virol 77 (Pt 6):1265-70, Snijder, E. J., and J. J. Meulenberg. 1998 (supra)). The highly polymorphic ORF5 gene encodes the major envelope protein GP5. The other major structural proteins include the M and N encoded by ORFs 6 and 7, respectively (Bautista, E. M., J. J. Meulenberg, C. S. Choi, and T. W. Molitor. 1996. Structural polypeptides of the American (VR-2332) strain of porcine reproductive and respiratory syndrome virus. Arch Virol 141:1357-65, Mardassi, H., B. Massie, and S. Dea. 1996. Intracellular synthesis, processing, and transport of proteins encoded by ORFs 5 to 7 of porcine reproductive and respiratory syndrome virus. Virology 221:98-112, Meng, X. J., P. S. Paul, and P. G. Halbur. 1994. Molecular cloning and nucleotide sequencing of the 3′-terminal genomic RNA of the porcine reproductive and respiratory syndrome virus. J Gen Virol 75 (Pt 7):1795-801, Meng, X. J., P. S. Paul, P. G. Halbur, and M. A. Lum. 1995. Phylogenetic analyses of the putative M (ORF 6) and N (ORF 7) genes of porcine reproductive and respiratory syndrome virus (PRRSV): implication for the existence of two genotypes of PRRSV in the U.S.A. and Europe. Arch Virol 140:745-55, Meng, X. J., P. S. Paul, P. G. Halbur, and I. Morozov. 1995. Sequence comparison of open reading frames 2 to 5 of low and high virulence United States isolates of porcine reproductive and respiratory syndrome virus. J Gen Virol 76 (Pt 12):3181-8, Meulenberg, J. J., J. N. Bos-de Ruijter, R. van de Graaf, G. Wensvoort, and R. J. Moormann. 1998. Infectious transcripts from cloned genome-length cDNA of porcine reproductive and respiratory syndrome virus. J Virol 72:380-7, Meulenberg, J. J., and A. Petersen-den Besten. 1996. Identification and characterization of a sixth structural protein of Lelystad virus: the glycoprotein GP2 encoded by ORF2 is incorporated in virus particles. Virology 225:44-51, Meulenberg, J. J., A. Petersen-den Besten, E. P. De Kluyver, R. J. Moormann, W. M. Schaaper, and G. Wensvoort. 1995. Characterization of proteins encoded by ORFs 2 to 7 of Lelystad virus. Virology 206:155-63, Meulenberg, J. J., A. P. van Nieuwstadt, A. van Essen-Zandbergen, and J. P. Langeveld. 1997. Posttranslational processing and identification of a neutralization domain of the GP4 protein encoded by ORF4 of Lelystad virus. J Virol 71:6061-7). GP5 is the main protein that induces neutralizing antibodies (Ostrowski, M., J. A. Galeota, A. M. Jar, K. B. Platt, F. A. Osorio, and O. J. Lopez. 2002. Identification of neutralizing and nonneutralizing epitopes in the porcine reproductive and respiratory syndrome virus GP5 ectodomain. J Virol 76:4241-50, Plagemann, P. G. 2004. The primary GP5 neutralization epitope of North American isolates of porcine reproductive and respiratory syndrome virus. Vet Immunol Immunopathol 102:263-75), although neutralizing epitopes have also been identified in GP3, GP4 and M (Meulenberg, et al. 1997 (supra), Plana Duran, J., I. Climent, J. Sarraseca, A. Urniza, E. Cortes, C. Vela, and J. I. Casal. 1997. Baculovirus expression of proteins of porcine reproductive and respiratory syndrome virus strain Olot/91. Involvement of ORF3 and ORF5 proteins in protection. Virus Genes 14:19-29). Extensive antigenic, genetic and pathogenic variations have been documented for PRRSV (Meng, X. J. 2000. Heterogeneity of porcine reproductive and respiratory syndrome virus: implications for current vaccine efficacy and future vaccine development. Vet Microbiol 74:309-29). The nucleotide sequence identity between the European (type 1) and North American (type 2) genotypes is only about 65% (Allende, R., T. L. Lewis, Z. Lu, D. L. Rock, G. F. Kutish, A. Ali, A. R. Doster, and F. A. Osorio. 1999. North American and European porcine reproductive and respiratory syndrome viruses differ in non-structural protein coding regions. J Gen Virol 80 (Pt 2):307-15, Nelsen, C. J., M. P. Murtaugh, and K. S. Faaberg. 1999. Porcine reproductive and respiratory syndrome virus comparison: divergent evolution on two continents. J Virol 73:270-80). Even among some isolates within the type 2 North American genotype and within the type 1 European genotype, the sequences differ by more than 10% (Forsberg, R., T. Storgaard, H. S. Nielsen, M. B. Oleksiewicz, P. Cordioli, G. Sala, J. Hein, and A. Botner. 2002. The genetic diversity of European type PRRSV is similar to that of the North American type but is geographically skewed within Europe. Virology 299:38-47, Meng, Paul, Halbern and Lum, 1995 (supra), Meng, Paul, Halbur, and Morozov, 1995 (supra), Stadejek, T., M. B. Oleksiewicz, D. Potapchuk, and K. Podgorska. 2006. Porcine reproductive and respiratory syndrome virus strains of exceptional diversity in eastern Europe support the definition of new genetic subtypes. J Gen Virol 87:1835-41, Stadejek, T., M. B. Oleksiewicz, A. V. Scherbakov, A. M. Timina, J. S. Krabbe, K. Chabros, and D. Potapchuk. 2008. Definition of subtypes in the European genotype of porcine reproductive and respiratory syndrome virus: nucleocapsid characteristics and geographical distribution in Europe. Arch Virol 153:1479-88, Stadejek, T., A. Stankevicius, T. Storgaard, M. B. Oleksiewicz, S. Belak, T. W. Drew, and Z. Pejsak. 2002. Identification of radically different variants of porcine reproductive and respiratory syndrome virus in Eastern Europe: towards a common ancestor for European and American viruses. J Gen Virol 83:1861-73). There exist at least 9 major clusters of PRRSV within the type 2 North American genotype, and 4 major clusters within the type 1 European genotype (Forsberg, et al, 2002, Stadejek, et al, 2002 (supra)).
Modified-live attenuated vaccines (MLVs) against PRRSV were generally effective against homologous strains but were less effective or ineffective against heterologous strains (Ansari, I. H., B. Kwon, F. A. Osorio, and A. K. Pattnaik. 2006. Influence of N-linked glycosylation of porcine reproductive and respiratory syndrome virus GP5 on virus infectivity, antigenicity, and ability to induce neutralizing antibodies. J Virol 80:3994-4004, Bastos, R. G., O. A. Dellagostin, R. G. Barletta, A. R. Doster, E. Nelson, F. Zuckermann, and F. A. Osorio. 2004. Immune response of pigs inoculated with Mycobacterium bovis BCG expressing a truncated form of GP5 and M protein of porcine reproductive and respiratory syndrome virus. Vaccine 22:467-74, de Lima, M., B. Kwon, I. H. Ansari, A. K. Pattnaik, E. F. Flores, and F. A. Osorio. 2008. Development of a porcine reproductive and respiratory syndrome virus differentiable (DIVA) strain through deletion of specific immunodominant epitopes. Vaccine 26:3594-600, Kwon, B., I. H. Ansari, A. K. Pattnaik, and F. A. Osorio. 2008. Identification of virulence determinants of porcine reproductive and respiratory syndrome virus through construction of chimeric clones. Virology 380:371-8, Lopez, 0. J., M. F. Oliveira, E. A. Garcia, B. J. Kwon, A. Doster, and F. A. Osorio. 2007. Protection against porcine reproductive and respiratory syndrome virus (PRRSV) infection through passive transfer of PRRSV-neutralizing antibodies is dose dependent. Clin Vaccine Immunol 14:269-75, Misinzo, G., P. L. Delputte, P. Meerts, C. Drexler, and H. J. Nauwynck. 2006. Efficacy of an inactivated PRRSV vaccine: induction of virus-neutralizing antibodies and partial virological protection upon challenge. Adv Exp Med Biol 581:449-54, Nilubol, D., K. B. Platt, P. G. Halbur, M. Torremorell, and D. L. Harris. 2004. The effect of a killed porcine reproductive and respiratory syndrome virus (PRRSV) vaccine treatment on virus shedding in previously PRRSV infected pigs. Vet Microbiol 102:11-8, Osorio, F. A., J. A. Galeota, E. Nelson, B. Brodersen, A. Doster, R. Wills, F. Zuckermann, and W. W. Laegreid. 2002. Passive transfer of virus-specific antibodies confers protection against reproductive failure induced by a virulent strain of porcine reproductive and respiratory syndrome virus and establishes sterilizing immunity. Virology 302:9-20, Papatsiros, V. G., C. Alexopoulos, S. K. Kritas, G. Koptopoulos, H. J. Nauwynck, M. B. Pensaert, and S. C. Kyriakis. 2006. Long-term administration of a commercial porcine reproductive and respiratory syndrome virus (PRRSV)-inactivated vaccine in PRRSV-endemically infected sows. J Vet Med B Infect Dis Vet Public Health 53:266-72, Zuckermann, F. A., E. A. Garcia, I. D. Luque, J. Christopher-Hennings, A. Doster, M. Brito, and F. Osorio. 2007. Assessment of the efficacy of commercial porcine reproductive and respiratory syndrome virus (PRRSV) vaccines based on measurement of serologic response, frequency of gamma-IFN-producing cells and virological parameters of protection upon challenge. Vet Microbiol 123:69-85). Commercial killed vaccines, except for farm-specific autogenous products, are not available in the U.S. The outcomes of the use of killed vaccines in other countries are not promising (Lager, K. M., W. L. Mengeling, and S. L. Brockmeier. 1997. Duration of homologous porcine reproductive and respiratory syndrome virus immunity in pregnant swine. Vet Microbiol 58:127-33, Misinzo, et al, 2006 (supra)).
It is thus important to develop broadly-protective and more effective PRRSV vaccines that would confer protection against a broad range of genetically diverse field isolates of PRRSV.